Surprised by Eifert's inability to take the field for this week's minicamp, a frustrated Marvin Lewis acknowledged Wednesday that the status of the 2015 Pro Bowl selection is uncertain for the start of training camp this summer.

"My understanding is he did tweak his back just a little bit," NFL Network Insider Ian Rapoport noted on Wednesday's edition of Inside Minicamp Live.

Lewis' frustration with Eifert's availability has been consistent throughout the team, per Rapoport. When the Bengals brought him back on a one-year deal, the hope was that he would be fully healthy and in prime condition for a bounce-back season that would earn him a lucrative long-term deal in 2019.

Plagued by a troublesome back injury that necessitated a third surgery, Eifert managed to play just two games last season. Through five NFL seasons, he's missed more regular-season games (41) than he's played (39).

When he's healthy enough to take the field, Eifert is a difference-maker for Cincinnati, providing quarterback Andy Dalton with one of the premier red-zone weapons in the league.